What made Mists of Pandaria Trademark believable for me? Here's some reasons:
Historically speaking this Trademark has always happened this way. Roughly 2 months before Blizzcon comes a Trademark. ONE trademark... and only one that goes up to Blizzcon. This has happened with Wrath. This has happened with Cata. This has happened with Pandaria. Why should now be any different?
If they're going to reveal it at Blizzcon, they had BETTER have a locked-in trademark by then. :P
2) Why psych-out the audience?
Why would Blizzard deliberately set up an expectation MONTHS in advance, just to disappoint fans for them to say "GOTCHA!!"?
You look at things like "Pandaria must be a joke!" but, to others, they're saying "Awesome! I was always hoping we could see the home of the Pandaren officially be brought into the game!". Not everybody will think like you and say "Ooooh! Demons! I was hoping for Pandaren - but that's obviously much better!". Because people's tastes are different than yours.
Why deliberately dash those hopes against the rocks?